This section introduces aspects that may facilitate better understanding of the present disclosure. Accordingly, the statements of this section are to be read in this light and are not to be understood as admissions about what is in the prior art or what is not in the prior art.
In the indoor cellular network construction, fine network dimensioning is highly costly or even impossible due to a limited construction budget and a complicated radio propagation environment. Usually, an area is covered by a number of sectors, each of which may be radiated by a decentralized antenna. These antennas may transmit identical or different radio signals. Without fine network dimensioning, these sectors may overlap irregularly on their boundaries. Thus, the Signal to Interference plus Noise Ratio (SINR) at boundary areas may be low when the antennas radiating these sectors transmit different signals at the same carrier frequency.
In order to guarantee a high SINR at the boundary areas, a technique called as combined-cell may be adopted. The basic idea of the combined-cell is to combine those sectors having an identical carrier frequency, an identical bandwidth and a partially overlapping coverage, to generate a common cell. The common cell covers the whole area under coverage of those sectors and is radiated by all of the antennas that radiate the sectors. In this common cell, any piece of time-frequency resource can only be allocated to one user equipment (UE); and all sectors provide the same Cell-specific Reference Signal (CRS) and Synchronization Signal, and the same public control channels. In Long Term Evolution (LTE) systems, the public control channels comprise Physical Broadcast Channel (PBCH), Physical Control Format Indicator Channel (PCFICH), Physical Hybrid-ARQ Indicator Channel (PHICH) and Physical Downlink Control Channel (PDCCH) in downlink and Physical Uplink Control Channel (PUCCH) and Physical Random Access Channel (PRACH) in uplink, whose characters, together with the CRS, are determined by a Physical Cell Identifier (PCI). In the common cell, all the sectors use the same cell ID.
FIG. 1 shows an example of a combined-cell system, in which a total of six sectors, which originally correspond to six independent non-combined cells with the same carrier frequency, are combined to form a common cell. Each sector is equipped with a separate antenna, and all antennas are connected to a common digital unit (DU) of a base station. All signal processing and protocol processing in both downlink and uplink is collectively executed at the DU, which makes it possible to jointly allocate resources and perform scheduling for the whole common cell.
A main benefit brought by the combined-cell is performance enhancement at the boundary areas, in which downlink signals from adjacent sectors strongly override. Since any piece of time-frequency resource can only be allocated to one UE, the sectors in the common cell may transmit identical downlink data channels to or receive uplink data channels from a UE simultaneously on the time-frequency resource allocated to that UE. In this way, the signals from the respective sectors are reinforced instead of interrupted with each other, and hence the downlink or uplink data transfer performance at the boundary areas are greatly improved.
Another benefit brought by the combined-cell is elimination of the UE handover when it is roaming among these sectors of the common cell.
However, these benefits are at the cost of reduced overall spectrum efficiency and system throughput, since one piece of time-frequency resource can only be used by only one UE, regardless of the location of this UE within the sectors constituting the common cell. Compared with the independent resource allocation at all the sectors without the cell combination, only a fraction of the spectrum efficiency is obtained.
An idea of spatial domain multiplexing (SDM) was proposed to compensate for the loss of the combined-cell. The SDM selects a group of UEs that are spatially isolated, and then allocates a same piece of time-frequency resource to them for data transfer simultaneously. This manner can enable multiplexing of resources of Physical Downlink Shared Channel (PDSCH) and Physical Uplink Shared Channel (PUSCH). However, it needs the multiplexed UEs to be scheduled jointly. This requires schedulers of multiple sectors to be coupled, which may greatly increase the implementation complexity and processing delay. Besides, the SDM has deteriorated performance for Release-8 UEs, because it cannot guarantee that the sectors whose CRSs are used in channel estimation and frequency offset estimation are just those who transfer downlink data to the UEs.
Furthermore, with the development of the radio communications technology, multiple carrier frequencies may be granted for a wireless network, in which signals may be transferred in the same sector but on different carriers. By combining the sectors at each carrier, multiple combined-cells may have the same or similar coverage areas on different carriers. Each carrier may run as an independent cell.
However, except for the general idea of cell combination and the SDM as introduced above, the combined-cell system with multiple carriers has not been optimized, e.g. for a balance between coverage and spectrum efficiency or capacity.